Copolymerization of polymers is generally chemically effected by grafting of the polymeric moieties at the sites of unsaturated bonds, with extensive cross-linking between the copolymers in a tight bond. However, as a result of the copolymerization, the resultant copolymers comprise structures which are more rigid than the original polymers, with reduced tensile strength, flow rate and capacity for elongation. The networks so formed in the prior art are generally of long chains with a high degree of crystallinity. The polymers are occasionally interlaced with one another or are cross-linked chains and are amorphous in nature, with properties which depend, not on their crystalline structure but on the interaction between the molecules.
During the polymerization processes of the prior art, additives, stabilizers, plasticizers and the like are used to improve the polymer performance but such improvement is limited by the compatibility between the various components of the polymer. Thus, for example, some additives and plasticizers form phase separation with the matrix of the polymer.
Various methods have been utilized in order to graft varying monomers to a polymer backbone. These methods include chain transfer reactions, hydroperoxidation, degradation (mechanical and thermal), functional group reactions such as redox initiation and condensation of molecules containing hydroxyl, carboxyl, amine, thiol and ester groupings. In order to effect the appropriate grafting to a polymeric backbone, in the prior art, a vinyl monomer or unsaturated moiety was necessary in the monomer to successfully attach or graft it to the polymer. Examples of such monomers, utilized in the prior art, include methyl methacrylate, styrene, methacrylic acid, unsaturated carboxylic acids, butadienes, unsaturated organic oils, vinyl chloride, acrylonitrile, maleic anhydride, acrylic acid esters, isoprene, divinyl ether, conjugated diolefins, polyamides, propylene terephthalate, polyethylene terephthalate, vinyl acetate, methacrylonitrile, isocyanates, polyureas, polyurethane, polythioureas, glycidyl methacrylate, etc.
The above exemplified materials and combinations result, for the most part, in incompatible or semi-compatible systems, when grafted to polymers and copolymers, such as plastics and elastomers. Because of such incompatibility, to whatever degree occurring, properties are reduced from that of homogeneous systems. The reduction in various properties is not predictable except to the extent that there is a pattern of reduction of tensile strength with increasing content of the graft chain.
A new trend for blending non-similar polymers is the generation of copolymers which can act as compatibilizers or functionalizing materials. Examples of such materials include maleic anhydride (unsaturated carboxylic anhydride) used in the grafting of nylon and other thermoplastics. With such method, peroxide, in the presence of vinyl groups or double bonds, initiates the grafting to polymers. The vinyl groups may be substituted by reactive groups such as (NH.sub.2), (CN), etc.
Another method is the grafting of polymers wherein a second polymer is formed during the grafting process. This, however presents problems not found with the grafting of pre-formed polymers. Specifically, the formation of the second polymer affects the mixing and distribution of the monomer in the polymer matrix, with the increasing of viscosity and lowering of monomer diffusion. This lowering of monomer diffusion also lowers the reaction rate by which the grafting moiety and the polymer will produce a copolymer which may function as an emulsifier to help impart homogeneity to the system.
With all such methods, basically the grafting has been effected by either polymerization of a vinyl containing monomer or an unsaturated material such as EPDM rubbers and maleic anhydride, wherein a free radical is produced by utilizing peroxides.